No question is too small for science, and there is a body of research devoted to determining the whos and hows of the asparagus smell of human urine. Here's a snapshot of said research.

For Marcel Proust, one of the after-effects of eating asparagus, was that it "…transforms my chamber-pot into a flask of perfume.'' This unusual phenomenon is well known by a (disputed) percentage of asparagus-eaters who are lucky (or unlucky) enough to be able to nasaly detect the vegetable's odorous metabolites in their (or others') urine. And it has been the subject of fairly intense scientific scrutiny for more than a century.

The first formal study came from Wilhelm Marceli Nencki, who, in 1891, published his paper - ‘Ueber das vorkommen von methylmercaptan in menschlichen harn nach spargelgenuss'. (Arch Exp Path Pharmak. 28:206–209) And, extraordinarily perhaps, to this day full agreement in the scientific community regarding the exact nature of the phenomenon is still wanting. For example, only a (disputed) percentage of people actually excrete the odorant(s), and the biological basis for the inability to produce the metabolite in detectable quantities is unknown. More perplexing still, as mentioned above, not everyone can smell the excreted chemical – even if it is present.

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Progress towards a full description is still being made however, and the latest edition of the journal Chemical Senses carries a research paper from the Monell Chemical Senses Center, Philadelphia, and the Department of Psychology, University of South Florida describing a new set of experiments which delve not only into the question of who is likely to excrete what (and in what quantities), but also who can or cannot detect the as yet unknown smelly chemicals.

Two experiments, involving a set of 38 adult male and female eaters and sniffers (recruited via a newspaper ad.) have made steps towards clarifying the enigma – confirming that there are genetic factors at work when it comes to being able to detect the odor. ("… people with a particular allele within an olfactory gene cluster is related to the ability to smell the odor.") There is, however, considerable scope for further work to be done on the asparagus/urine-odor/enigma :

"… the recognition of the asparagus odor in urine is not an all-or-nothing phenomenon; some people produce an asparagus odor that is easy to detect, and the presumption is that some people produce more odorant. However, because the odor-causing molecules have not been unequivocally identified,[…] it is not possible to measure its concentration in urine; it is reasonable to assume that odorant production varies from individual to individual, and people with urine that does not have a detectable odor may produce it, albeit at a low concentration."

The full paper: ‘Excretion and Perception of a Characteristic Odor in Urine after Asparagus Ingestion: a Psychophysical and Genetic Study' is available in .pdf format here.